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Interpretive Summary: In the agricultural Midwestern U.S., fertilizer application supplies crops with vital nutrients for growth, including nitrogen (N) and phosphorus (P). However, fertilizer nutrients that are applied in excess of crop needs can remain in the soil profile and are susceptible to loss, via surface runoff or subsurface drainage, during precipitation events. Excess N and P that leak from agricultural lands can then enter adjacent streams and be transported to downstream water bodies where they fuel algal blooms like those that occur in Lake Erie and the Gulf of Mexico each year. However, managing fertilizer application according to the principles of the 4R's (i.e., applying the right fertilizer, at the right rate and time, in the right place) can influence the amount of N and P that accumulate in agricultural soils and may subsequently impact N and P losses. The goal of our study was to examine the relationship between fertilizer application and nutrient losses (both N and P) across 40 monitored agricultural fields in northwestern Ohio with different fertilizer management practices. We found greater accumulation of N and P in agricultural fields with greater fertilizer application rates. We also found agricultural fields that received organic and mixed (organic plus inorganic) fertilizers accumulated more N and P than fields receiving inorganic fertilizers alone. Overall, increased application rates and use of organic fertilizer led to greater N and P losses from agricultural fields. These findings are important for scientists, producers, and resource managers because they link N and P losses to specific fertilizer management practices, primarily application rate. The techniques employed here, which compared fertilizer inputs to nutrient outputs (i.e., losses), can be used as a simple accounting tool to help producers and managers better match fertilizer application to crop needs. Our results also demonstrate that N and P losses represent a small portion of the total amount applied to farm fields, thus additional management practices are likely promoting nutrient retention.

Technical Abstract: Excess nitrogen (N) and phosphorus (P) applied to agricultural fields may be transported to adjacent surface waters and exported to downstream systems, leading to environmental issues including algal blooms. Management practices associated with fertilizer application, including the input rate and source (e.g., inorganic vs. organic), influence N and P accumulation within agricultural fields and may ultimately determine N and P loss in surface and subsurface hydrologic pathways. In this study, we used management data provided by individual farmers to quantify annual agronomic N and P balances (i.e., budgets) across 40 monitored agricultural fields in the Western Lake Erie Basin for a total of 150 site years. We examined the effect of application rate and fertilizer source on agronomic N and P balances, as well as determined the relationship between agronomic N and P balances and hydrologic N and P losses. We found that N and P balances increased with application rate and varied among fertilizer sources; fields receiving mixed or organic fertilizers had greater N and P balances compared with inorganic fertilizers. Both hydrologic N and P loss increased with agronomic N and P balances, demonstrating that nutrient accumulation within agricultural fields generally increases nutrient loss. Additional controls on hydrologic N and P losses varied, with hydrologic N loss strongly controlled by total discharge while hydrologic P loss differed among fertilizer sources and indicated legacy P in some fields. These results highlight that both nutrient and water management practices are likely necessary to achieve water quality improvements in downstream lentic and coastal ecosystems.